Backfiring through the exhaust?

[type11969]

Sometimes during deceleration, but more frequently during light throttle applications, I am getting backfiring through the exhaust. Curious what people think the causes might be. I at first thought it was an exhaust leak . . . but I resealed the system and there was no change at all in the amount of backfiring. I need to go through and check the timing, perhaps it fell out of adjustment, but I am running a unilite and I will be suprised that it changed in such a short amount of time (~400 miles since the last timing check). Any other ideas?

-Chris

[MASSIVE TYPE IV]

tight exhaust valve??
spark plug trying to foul (firing late)
intake leak
partially clogged carb jet/circuit
bad ignition wire/dizzy cap/rotor

thats where to start!

[type11969]

Tight exhaust valve - just installed and adjusted Porsche adjusters, shouldn't be a problem

Spark plugs fouling - just replaced, running CDI and unilite so should be pretty robust

Intake leak - will have to check, although doesn't seem to be running hot or lean

Partially clogged circuit - car stumbles during the tranisition from idle to mains, could be a cause

Wires just were replaced, unlite dist is basically brand new, so hopefully none of those are at fault.

Thanks for the help.

-Chris

[MASSIVE TYPE IV]

New parts should ALWAYS be suspect FIRST!!

Never let "New" fool you, especially with ignition components!

Double check all of them, but I'd lean toward a carb issue..

[K70]

Fixing the same problem for one year.
No chance of getting ^ to answer sadly.
Share your wisdom -
Bring out a type 4 book please.

[Ol'fogasaurus]

Something like the backfiring can be caused by many things such as fuel getting into the ignition or exhaust system the igniting due to any of several ways. Running rich (firing late) along with timing or wrong parts can cause late or even early firing in the system.

Chris gave you some good ideas on what to check for. Also check the valve settings and/or valve seats. I've seen valves not sitting straight in the guides hence the seat or the seats could be bad.

So many things that can drive one crazy isn't there.

Lee

[K70]

Cheers,
I did the valve check, with oil flush and change.
No joy!
Loose cam gear?
Flat cam?
Damaged timing gear?
Why is compression over spec (135>150) stock engine?
No noise or smoke or oil burnt.

[K70]

Timing is 28*@3500rpm.
Everything bolted outside engine swapped or tested.
Crazy?
Not even close....!

[Ol'fogasaurus]

Just for giggles... is the cam installed/aligned/ground correctly (et al)?

Seen it happen before.

Lee

[K70]

When the distributor stopped at an opportune place,
It was easier to see the hall sender shutter too close to wiring and coupled with the shaft wear creating a perfect **** storm.
The aftermarket quality d**** hall sender unit (only one available at the time) is so loose and wobbly a field mouse sneezing can move it.
They Should stick to bacon not engineering/manufacturing .
What engine damage it caused is to be found out.
So starting a post on distributor rebuilding and upgrading the #071-905-205 distributor to suit the DRLA carbs.

[K70]

Apparently the Mr. Raby used Mallory - until they stopped making it properly.
So I'd need an old one if you have a spare one pls...

[type11969]

You might want to consider a 123 programmable distributor

[Typ 4]

You might want to look at the fuel pressure and needle/seats as well as setting floats on your DRLA's. When they get old (in high mileage) they can start to leak at even low pressures. Check idle and main jetting as well as emulsion size. Got DRLA's too and over fuelling was the issue in mine.

[K70]

Thanks type 4.
I am at about 2.5 psi on fuel.
Spark Plugs dark tan to grey.
Just tried out a new distributor, a waterboxer copy/svda.
It is on 8 degrees btdc idle /28 degrees all in.
Bottom end great, torquey.
No backfiring.
No overheating so far.
But about 72mph on flat road is max, so would like to see if a tiny bit of improvement can be found???
Any distributor upgrades to curve etc?